In the early development of data display systems, it was customary to employ a cathode ray tube wherein a layer of phosphor material was made to luminesce by means of an electron beam that scanned across the layer of phosphor material. Although good quality images can be created in this manner, the physical size of the images that can be created by a cathode ray tube is severely limited by various factors, such as, for example, power required and distortion of the electron beam path. In order to provide images of greater size, numerous schemes have been proposed for optically enalrging an image created by cathode ray tube. Although an image of greater proportions can be obtained in this manner, the amount of light available from the phosphors present on the face of the tube is very limited. As a result, the quality of the enlarged image and particularly the brightness thereof has heretofore been very poor.
Cathode ray tubes have also been utilized in systems which employ a projection system having a light source which is independent of the cathode ray tube light emission. For example, in U.S. Pat. Nos. 3,667,830; 3,701,586; 3,609,222; and 3,746,785, large scale displays are provided by utilizing a display structure having a deformable, light reflective metallic film supported by a support grid which is situated within a cathode ray tube. When an electron beam scans across the display structure, charge accumulates on areas of the display structure in accordance with the information content of the electron beam. This charge accumulation causes small deformations or dimples to form in the metal film at the areas of the charge accumulation. When light from a projection system is directed upon the metal film, only light which strikes the deformed areas reaches a display screen. Thus, a light image is formed on the display screen corresponding to the dimpled image formed in the metal film by the electron beam. A flood gun must be provided within the cathode ray tube to neutralize the charged areas to thereby allow the deformations in the metal film to relax to the non-deformed or normal position.
The size limitations of display systems using cathode ray tubes has led to the use of matrix addressed displays when large displays are required. In a matrix display, pairs of conductors in a two-dimensional array of such conductors are utilized to address each elemental area of the display to thereby initiate the emission of light at a selected elemental area when the pair of conductors associated with that selected elemental area are properly biased. In such a display system, as described in U.S. Pat. No. 3,091,876, when a selected elemental area is properly biased, a pressure valve is opened which forces a portion of a flexible membrane out past the end of a tubular support member. In this outer position, the reflective surface of the membrane refelcts incident light to provide a visible "bright spot" in the surface of the display to thereby provide a visible display. U.S. Pat. No. 3,091,876 also teaches using an electroluminescent panel in conjunction with the flexible membrane whereby when an elemental portion of the membrane is addressed, the membrane is forced by a pressure system into contact with an electrode of the electroluminescent panel whereby a voltage is applied across an elemental area of the electroluminescent panel to thereby cause it to emit visible light.
As noted, the display systems described which utilize cathode ray tubes suffer from size limitations and the expensive cathode ray tube component. Also, these systems do not use ambient light as the projection light source. The matrix display systems that utilize a pressure source also suffer from the requirement of expensive mechanical components and also, especially when using an electroluminescent panel, from the lack of a threshold behavior since the elemental areas adjacent a selected elemental area receive half the voltage applied across the selected elemental area and that voltage may be sufficient to initiate undesirable glow discharges and hence undesirable light output at areas adjacent the selected elemental area. Also, many of the display systems described do not have machine readable capabilities.
A display device which is an improvement on the above mentioned prior art devices and which provides a flat optical display device that uses ambient light, is addressed by existing electronics, has a threshold behavior and is machine readable is disclosed in U.S. Pat. No. 4,035,061, issued July 12, 1977 and assigned to the assignee of the present invention.
In particular, an array of focusable image elements in conjunction with a corresponding array of optical stops or filters, one optical stop or filter for each image element are utilized. Each of the focusable image elements is addressed in a matrix manner whereby a force can be created at selected image elements to change the focal point of the light reflected or transmitted by those selected image elements. The change in the focal point of the selected image elements causes some of the ambient light reflected or transmitted by those image elements to bypass the optical stops associated with those image elements which optical stops would otherwise absorb most, and desirable all, of the light reflected from or transmitted by those elemental areas had the focal point of those focusable image elements not be changed. Thus, the change in the focal point of selected image elements or areas of the display is used to provide a visible display which utilizes ambient light and existing switching technology. In addition, a sharp threshold behavior is exhibited which enables the number of peripheral address elements to be held to a manageable quantity. In one of the disclosed embodiments, the focusable image elements are light reflectors which are provided over a perforated support sheet having a perforation for each image element. The reflectors can be spherical or paraboloidal indentations in a flat sheet, suitably coated for optical reflectivity and electrical conductivity, and stretched over the perforations of the support sheet. The indentations are "popped" between concave and convex curvatures to provide the desired change in focal point required for selected image elements to provide a display.
It would be desirable if the basic display device disclosed in the aforementioned U.S. Pat. No. 4,035,061 could be modified to provide additional video display techniques while retaining the aforementioned advantages thereof.